This invention relates to a device for the knotless joining of threads and yarns using compressed air. These devices, commonly known as "thread splicing devices" are much used for joining threads and yarns, and have largely replaced the previously used knotting devices because of their numerous advantages over these latter.
It has been sought to continuously improve pneumatic splicing devices in order to make them suitable for an increasingly wide range of threads and yarns, including natural fibre, artificial fibre, monofilament, short fibre and long fibre types, to in each case achieve knotless joints of high tensile strength and of good appearance, so that they differ as little as possible from the strength and appearance of the original continuous thread or yarn. Various systems and methods are used to achieve this knotless joining. Pneumatic splicing is always carried out in a splicing chamber (also called a splicing channel) into which, after suitable prior pneumatic or mechanical treatment for removing the original twist of the threads and for opening and parallelizing their fibres, the ends of the threads to be joined together are inserted so that they lie partly superimposed one close to the other and are then subjected to short strong compressed air jets fed into the chamber through variously arranged apertures or nozzles.
One of these systems aims mainly at obtaining thorough mixing of the fibres forming the ends of the threads or yarns to be joined together so that they become effectively tangled and interlaced. To implement this system splicing chambers are used having substantially V or U cross-sections into which the compressed air jet or jets are fed from the base to directly strike the ends of the threads to be joined. As the chamber is in communication with the outside both at its two ends or lateral openings and via the front opening of a longitudinal slit through which the threads are inserted into the chamber, it is well known to close this front opening and hence the chamber during the splicing operation by a cover to prevent the thread ends escaping frontally through the opening of said longitudinal slit by the effect of the compressed air jets which strike them. In this case, under the effect of the jet or jets fed from the base of the chamber, the thread ends undergo violent undulatory action to strike the cover and the chamber base, so that their fibres are effectively interlaced to achieve a good strong knotless joint. However the appearance of the joint obtained suffers from the fact of individual fibres projecting from the sides of the joint.
It has been found that the result is much better if a substantially V or U-shaped chamber is used rather than a circular cross-section chamber, especially if not provided with a cover for the temporary closing of the front opening of its thread insertion slit.
Another system used aims more at mutual rolling-together of the thread ends to be joined, and uses mainly circular cross-section splicing chambers with compressed air feed nozzles arranged to cause rotary air circulation. In this case normally at least two opposing nozzles are used opening substantially tangentially into the circular chamber at points more or less close to the two ends or side openings of the chamber. This system, by which the joints are of excellent appearance because the end fibres of one thread are properly rolled about the other thread but are of less mechanical strength, requires that in arranging the nozzles by which compressed air is fed into the chamber, account be taken of the original twist of the yarns in the sense that they do not cause them to untwist but instead act in the same direction as their twist. The nozzle arrangement has therefore to be changed in changing from S to Z twist yarns to be joined. These briefly described known systems hence each has its advantages and defects.
It was therefore apparent that a system would be sought which combines all the advantageous of those previously used while at the same time eliminating their drawbacks. Attempts in this direction have been made known from the documents DE 32 40 485 A1, DE 34 11 482 A1 and DE 40 19 959 A1.
According to document DE 32 40 485 A1, a splicing device comprises a circular chamber with an oblique slit for inserting the threads to be joined, with two compressed air feed nozzles opening into the centre of the chamber from opposite sides, the first of these nozzles opening into the chamber from one side of said slit and being directed towards the longitudinal axis at the centre of the chamber and against its opposite wall, whereas the other nozzle opens into the chamber from the opposite side of the slit and is directed substantially tangentially to the circular chamber, the first nozzle being connected by a shorter duct and the second nozzle being connected by a longer duct to a common compressed air feed duct. The longitudinal slit through which the threads are inserted into the chamber is again open frontally and in communication with the outside. On feeding compressed air through said feed duct an air jet initially leaves from the first nozzle to interlace the fibres, and a moment later a supplementary air jet leaves from the second nozzle to roll the ends of the threads. The splicing operation takes place in two successive stages with a small delay between the first and second stages due to the different lengths of the ducts by which the compressed air is fed to the two nozzles, air being fed into the chamber firstly only from the first nozzle, and afterwards from both.
According to document DE 34 11 482 A1, the splicing device is improved in the sense that three nozzles are provided for feeding compressed air into the chamber (which is again of circular shape with a frontally oblique longitudinal slit always open for the insertion of the threads), of which one is positioned at the centre of the chamber and is directed towards its longitudinal axis and opposite wall, and the other two nozzles oppose each other and are displaced towards the respective ends of the chamber to open into it substantially tangentially. Compressed air from a single feed duct is fed to the three nozzles via a duct which partly surrounds the circular chamber. In this case the purpose of the central nozzle is to cause fibre interlacing and the purpose of the two opposing lateral nozzles is to roll the ends of the two threads in opposite directions. The arrangement of these lateral nozzles has to be determined on the basis of whether the threads to be joined are of S or Z twist.
Document DE 40 19 959 A1 proposes a further system improvement, consisting substantially of providing to the sides of the slit, through which the threads are inserted into the centre of the chamber, a first two opposing nozzles directed towards the longitudinal axis of the chamber and against its wall opposite said slit, and a further two mutually opposing nozzles displaced towards the respective ends of the chamber and opening into it substantially tangentially. The first two nozzles are connected by a common channel to a first compressed air feed duct and each of the further two nozzles is connected by its own channel to a common second compressed air feed duct. In this manner the order of compressed air feed into the chamber can be diversified so that it occurs firstly only through the first two nozzles and a moment later only through the further two nozzles.
These splicing devices proposed in the three aforesaid documents each have a chamber of circular cross-section constant from one end to the other, the chamber being in communication with the outside not only via its lateral openings but also frontally via the frontal opening of the always open slit through which the threads are inserted and which extends for the entire length of the chamber.
It has now been found that splicing devices of this type still suffer from various drawbacks deriving from the aforedescribed shape of the chamber. Firstly, the fibre interlacing achievable with the nozzle or nozzles directed towards the longitudinal axis of the circular chamber is not perfect in that it is not possible for the air jets leaving these nozzles to transmit the violent beating action and undulating movement to the two threads by which the fibres mutually mix, tangle and interlace. This is because the compressed air can also leave the chamber through the frontal opening of its slit through which the threads are inserted, so losing its effectiveness, and in addition the air jets leaving the nozzles directed towards the longitudinal axis of the chamber must always be directed against a solid opposite wall of the chamber and never against the front opening of said insertion slit, otherwise the threads are expelled from this opening.
In addition, the air jets fed substantially tangentially into the chamber have the same constant uniform cross-section over their entire length, and can therefore interfere with each other to make the rolling of one thread end about the other uncertain.